In a wind turbine generator, a hub equipped with wind turbine blades rotates in response to wind energy, and a gear box increases the speed of this rotation so as to drive a generator, whereby power is generated.
The aforementioned hub is attached to an end of a yawable nacelle that is set on top of a wind-turbine tower, and is supported in a rotatable manner about a rotation axis extending substantially in the horizontal direction.
In order to control the output power in accordance with the wind speed, such a wind turbine generator performs pitch control for changing the angles of the wind turbine blades.
Known examples of pitch-control devices in the related art include a hydraulic pitch-control device that utilizes hydraulic pressure and an electric pitch-control device that uses a motor (for example, see Patent Literature 1).